Natural gas, that is a mixture of methane and small amounts of higher molecular weight hydrocarbons from gas and oil wells, often contains a substantial portion of nitrogen. In situ under high formation pressure, hydrocarbon gases are either compressed into liquids or are dissolved in the heavier liquid hydrocarbon fractions. When natural gas is recovered from oil wells primarily concerned with oil production, the gases are said to be "associated" with the liquid fractions. As pressure is released during recovery, compressed gases or dissolved associated gases, are released to form a gas at the well head which, when free of excessive quantities of nitrogen and other contaminants, is suitable for being processed and used or sold as fuel or chemical feedstock. The nitrogen may occur naturally and/or may result from gas injections used to enhance oil recovery. In such oil recovery enhancement, nitrogen is injected at selected locations in an oil field formation to drive otherwise unrecoverable oil to the production well or wells. As the wells age, the nitrogen constituent of associated gas can increase up to 80 mole % or more of the total associated gas recovered from the well. When the nitrogen content exceeds 5 mole % or more of the natural gas, the heating value or chemical feedstock value of the gas is reduced, and the cost of gas compression, gas transport, and other gas handling is greatly increased relative to the usable portion of the gas.
The prior art contains many processes and apparatus for reducing the portion of nitrogen and other contaminants in natural gas in order to increase the value of the gas and to reduce costs. Generally the prior art removes the non-nitrogen contaminants and separates the higher boiling hydrocarbon components from the low boiling components consisting of nitrogen and methane prior to using one of two techniques, cryogenic condensation or cryogenic absorption, to separate methane from nitrogen. The separation processes are desirably performed at the highest practical pressures, well head pressure or pipeline pressure which can be up to 1,090 psia (75 bar), in order to reduce compression costs resulting from pressure reductions necessary for the separation processing. Minimizing refrigeration loads and fluid compression loads are likewise considered important to achieve ma imum efficiency.
Cryogenic condensation techniques for condensing methane from a gas mixture of methane and nitrogen employ refrigeration units or "cold boxes" to produce condensation temperatures, and employ distillation type apparatus to obtain maximum conversion and separation of gaseous nitrogen and liquid methane. These prior art condensation processes are generally limited to operation at a maximum pressure of about 400 psia (27 bar) since the efficient formation of separable liquid and gas phases occur well below the critical pressure which, in the nitrogen and methane mixture, is in the range from 500 to 730 psia (34 to 50 bar), with the minimum being on the pure nitrogen side. This limitation with a high input pressure, results in increased compressor duty to bring the methane back up to pipe line pressure. Also the prior art required the lowest effective temperatures above the boiling point of nitrogen for efficient condensation. Typically, condensation temperatures below -240.degree. F. (-150.degree. C.) are employed. Refrigeration duty to achieve such temperatures is sufficiently high to constitute a substantial cost factor in the separation. Additionally, some nitrogen condensation occurs at these temperatures resulting in significant limitations upon the maximum rectification and separation of methane and nitrogen that can be achieved by cryogenic distillation. Further, traces of carbon dioxide present in the associated gas may freeze out at these low temperatures, resulting in clogging of the condensation and distillation apparatus.
Cryogenic absorption processes, as exemplified in U.S. Pat. Nos. 2,603,310 and 2,744,394, employ liquid absorbents such as ethane, propane, propylene, ethylene, and pentane to preferentially absorb methane from the gaseous mixture of methane and nitrogen. This absorption is generally performed in an absorption tower at a relatively high pressure and with a cooled absorbent, and then the absorbed methane is released in a demethanizer or regenerator tower by heating the circulating stream of liquid absorbent at a reduced pressure. Absorption processes have the advantage that separation occurs at a much higher temperature, e.g., -40.degree. to -70.degree. F. (-40.degree. to -55.degree. C.), compared to condensation processes. Additionally higher processing pressures can be employed; the above mentioned U.S. Pat. No. 2,744,394 discloses that a pentane absorbent increases the critical pressure to enable producing an overhead nitrogen-rich gas stream, a side methane-rich stream and a bottom liquid pentane with absorbed methane stream at pressures exceeding 1,000 psia (69 bar). However, absorption processes require a high absorbent circulation rate to attain a sufficiently high methane/nitrogen separation. This high absorption circulation rate results in relatively high capital and operating cost caused by a high refrigeration requirement for the absorbent, high pumping horsepower, large fluid handling equipment, and substantial absorbent loss.